Organic electroluminescence devices (also referred to as “organic EL devices”) is a spontaneous light emitting devices which utilize the phenomenon that a fluorescent or phosphorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of laminate type which can be driven under low electric voltage was reported, many studies have been made on organic EL devices which are made of organic materials. In the reported laminate devices, tris(8-quinolinolato)aluminum was used in the light emitting layer and a triphenyldiamine derivative was used in the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excitons by recombination can be increased, because the electrons injected from the cathode are blocked, and that the excitons formed in the light emitting layer can be confined. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting, light emitting layer as employed in the reported device and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the emitting material of organic EL devices, for examples, chelate complexes, such as tris(8-quinolinolato)aluminum, coumarin derivatives, tetraphenylbutadiene derivatives, distylylarylene derivatives, and oxadiazole derivatives have been known. It has been reported that these emitting materials emit blue to red visible lights and color devices are expected to be developed.
The fluorescent emitting material which causes the emission from singlet excitons has been hitherto used as the emitting material of organic EL devices. Recently, in addition to the fluorescent emitting material, it has been proposed to use the phosphorescent emitting material which causes emission from triplet excitons. The singlet excitons and the triplet excitons may generate in a ratio of 1:3 in organic EL device by the recombination of electros and hole in accordance with their difference in the spin multiplicity. Therefore, an organic EL device employing the phosphorescent emitting material achieves an emission efficiency three to four times higher than that of an organic EL device employing only the fluorescent emitting material.
However, a high efficient and long lifetime blue-phosphorescent emission is difficult to achieve. Therefore, the host material, the hole transporting material, and the electron transporting material which are to be exist around the phosphorescent dopant are required to have a high triplet excited energy for achieving a high internal quantum efficiency of emission, to have a wide gap for energetically confining the excitons of the emitting dopant, to have a high carrier injecting ability and transporting ability for achieving a high efficiency of power conversion, to allow a driving at low voltage, and to have a high chemical and heat stability for a long lifetime. The device performance may be optimized by optimizing the chemical structure of material. If the performance (mainly carrier balance) is simply changed according to the structural modification of the material, the study on the material can be significantly facilitated.
Patent Documents 1 and 2 disclose a compound having a carbazole ring. However, a compound having an unsaturated nitrogen atom in the ring is not disclosed.
Patent Documents 3 to 5 disclose a dibenzofuran ring and a dibenzothiophene ring each having an unsaturated nitrogen atom in the ring and a combined structure of carbazole rings wherein the nitrogen atom at 9-position of one ring is bonded to the carbon atom at 3-position of another ring. It is reported that a compound having such structure reduces the oxidation-reduction potential and improves the electrochemical stability. However, these patent documents are silent about the properties of a compound which has a dibenzofuran ring or dibenzothiophene ring each having an unsaturated nitrogen atom in the ring and the performance of a device employing such a compound.
Patent Documents 6 discloses a compound which has a dibenzofuran ring or a dibenzothiophene ring each having an unsaturated nitrogen atom in the ring, wherein these rings are bonded to each other by carbon atoms through a linking group.